Allow reopening datasets

https://redmine.teklia.com/issues/7725

While trying to make datasets strictly immutable is useful, it causes more trouble than it's worth.

PartialUpdateDataset and UpdateDataset should support updating the state from complete or error back to open, without requiring Ponos task authentication. When this happens, the task_id must be automatically set to None, to dissociate any previously built artifacts and allow rebuilding them. This behavior should be documented in the help_text for state.